Closure elements of this type are used as mass-produced items in bores that have different diameters and internal pressures. A wide range of embodiments are produced depending on the size of the bore and the strength of the effective internal pressure. Some of these embodiments are disclosed in DE AS 1 120 415 and U.S. Pat. No. 6,708,978 B2. The closures described here are made as one-part closure discs. They are therefore easier to produce than multi-part closures, such as for example the closure described in WO 2009/000317 A1. Since however the closure discs have relatively small dimensions, they are often cumbersome to handle. In addition, the closure disc must be supported against an internal counter-surface of the bore during the installation process. As a result of this the bore must be precisely graduated in order to ensure that the closure disc has the exactly intended installation depth in the installed state.